I think I know what you mean. However, have you considered importing a .png image such as this . . . >> << . . . then after creating your Macro, changing the icon to something like the one I have shown.
I created 'your Macro' and produced this below with the new assigned button:
I have never used OpenOffice before and downloaded it to try this method for you. It maybe long winded, but it works:
Firstly, Right click and Save the Euro button image I have showed in this post to your Desktop.
OpenOffice.org Writer > Tools > Macros > Record Macro > Start recording >Press the Ctrl + Alt + 4 keys together > Stop recording.
Name the Macro say 'Euro' > Save
Now go back to Tools > Macros > Organise Macros > OpenOffice.org basic > Expand 'My Macros' > Module 1 > Euro (right pane) > Assign > Toolbar content > Add > Category > Scroll down to OpenOffice.org Macros > Expand till you see 'My Macros' > Module 1 > Euro (right pane) > Add
You should have Button on the Toolbar with the wording 'Euro' on it. Now go to View > Toolbars > Customize > In Toolbar content, Highlight 'Euro' > Modify > Change Icon > Import > Navigate to Desktop > Select the Pasted 'Euro' .png file > Open > Highlight the new Icon > OK/OK
The new Euro button appears in the Toolbar for you to use when you require.